Organic solvent solution thermosetting coating compositions containing polyester resins in combination with aminoplast resins are well known, but these normally contain a low proportion of resin solids because the polyester resins commonly used provide excessive solution viscosity when the resin solids content of the system is increased.
One way to minimize the emission of organic volatiles from a coating operation is to increase the resin solids content of the coating solution, thus directly decreasing the amount of volatile organic solvent needed to deposit a given quantity of coating solids. Since this cannot be done with the polyester resins commonly used, an effort has been made to find new polyester resins which will possess reduced solution viscosity enabling an increase in the solids content of the coating.
Another objective in the coating industry is to minimize the baking temperature. The polyester resins commonly used require baking temperatures of about 300.degree. F. and higher, and it is desired to minimize the baking temperature to conserve energy to minimize cost, and to enable the coating of heat-sensitive thermoplastic substrates. When one employs a lower molecular weight polyester in an effort to increase the solids content of the solution coating composition, then a more extensive reaction is needed in order to build the film quality which is needed in the final cured coating. The effort to increase solids content is thus made more difficult by the need to maintain coating performance using minimal baking conditions for cure.
The provision of an appropriate polyester resin to solve the industrial problem outlined above and to thereby enable thermosetting solution coating compositions of the type under consideration possessing increased solids content combined with high reactivity and the capacity to provide cured films of good quality using minimal baking conditions is thus the prime objective of this invention.